HECRAS meandering stream: XS/cutlines intersect

[VeLuMa]

Dear forum members,
I am a newbie in working with HECRAS and I would be happy to hear your advise.
Please find a picture of the situation attached and here is my problem:
The stream is non perennial, only 1-2 m wide and strongly meandering. I have now created cross sections 200m wide at an interval of 100m.
My questions are now:
Is the interval of 100m too coarse (I think so)
Does the whole cross section have to be perpendicular to the flow? Or can I make cross sections that are perpendicular to the flow only in the area of the channel and then the cross section deviates (snaps off) so that they do not intersect with each other?
What would you propose how to handle the intersecting cross sections?
Thank you very much for your help!

 

[gbam]

First off are you planning on modeling a major flood event? If so, your xsecs will not be perpendicular to the main low flow channel. They will be perpendicular to flow including overbank flow. Meanders may be gone during a high flow event. Your picture does not describe the overbank condition.

 

[VeLuMa]

Thank you for your reply. Yes, I am planning to model a major flood event. So how must I creat the cross sections?

 

[Rye1]

It is probably easier to hand draw cross sections - your cross sections seem to be auto generated. RAS sections should be perpendicular to the flow and should be "dog eared" to follow the most appropriate path (depending upon engineering judgement). The spacing of the sections also depends upon engineering judgement up to a point. In a long flat floodplain, I would not space sections further than 150 m. Around bends or tight areas I may space the sections as close as 15 m.

Robert Billings

http://www.newrivereng.com

 

[VeLuMa]

I just learned that the cross sections must be perpendicular to the flow in the channel as well as perpendicular to the flow on the floodplains = dog legged. Is that the same than dog eared?

 

[Rye1]

Yes, dog legged is probably the best term to use.

Robert Billings

http://www.newrivereng.com

 

[gbam]

Be careful when cutting your XSECs if your main channel only contains low flow events (2-Year). I have always looked at the flood plain width and started with that to set my XSECs and somewhat ignored the low flow channel especially if your floodplain looks like this:

--------------\_/-------------------------------

It will be an iterative process.

 

[francesca]

Read the Hydraulic Reference manual please. It is quite clear on how to cut cross-sections and many other things you need to consider to build a decent model.

There seems to be no rhyme or reason for the cross-section locations you have cut. When I have a meandering stream, I cut my sections through areas where the channel banks are parallel to the center of gravity flow direction and use my channel Manning's to reflect the meandering channel. Be aware that if your channel flips from left to right in your overbanks, it will not give you a great model. HEC-RAS divides flow up into channel and left/right overbank flow. When there is a large change in conveyance from one cross-section to another you get a lot of errors and conservative results.

 

[VeLuMa]

Thank you very much, Francesca! The channel I am modelling is meandering so strong that the dog-legging does not seem feasible and I will try to use your advise and compare.